The present invention relates to a process for separating (removing) p-dichlorobenzene (hereinafter referred to as p-DCB) from an isomeric mixture of dichlorobenzenes, and particularly to a process for separating (removing) p-DCB from a mixture of m-dichlorobenzene (hereinafter referred to as m-DCB) and p-DCB. More in detail, the present invention relates to a process for separating (removing) p-dichlorobenzene from an isomeric mixture of dichlorobenzenes, comprising adding an eutectic agent of 1-bromo-4-chlorobenzene (hereinafter referred to as BCB), 1,4-dibromobenzene (hereinafter referred to as DBB) or a mixture of BCB and DBB to the isomeric mixture, heating the thus formed mixture to dissolve thereof partly or wholly, and cooling the thus formed solution, thereby crystallizing eutectic crystals (co-crystals) of p-DCB and the eutectic agent in order to separate p-DCB from the isomeric mixture of dichlorobenzenes.
The process according to the present invention is particularly useful industrially as a process for economically separating p-DCB from a mixture of m-DCB and p-DCB to obtain highly pure m-DCB (purity: higher than 95%).
The isomeric mixture of dichlorobenzenes is obtained by chlorinating benzene or monochlorobenzene. The composition of the isomeric mixture of dichlorobenzenes depends largely on the kinds of the catalyst and the reaction conditions of chlorination, however, in ordinary cases, the mixture consists essentially of from 55 to 80 % by weight of p-DCB, from 0.01 to 1% by weight of m-DCB and from 20 to 45% by weight of o-dichlorobenzene (hereinafter referred to o-DCB).
By cooling the reaction product to from -20.degree. to 10.degree. C., most of p-DCB therein crystals out therefrom, and by filtering the thus obtained crystalline slurry, most of p-DCB is removed to obtain crude o-DCB consisting essentially of from 60 to 85% by weight of o-DCB, from 15 to 40% by weight of p-DCB and from 0.02 to 2% by weight of m-DCB. The above-mentioned steps have been ordinarily carried out industrially.
By distilling most of p-DCB and m-DCB off from the thus obtained crude o-DCB, purified o-DCB consisting essentially of from 80 to 99% by weight, from 0.1 to 20% by weight of p-DCB and from 0 to 1% by weight of m-DCB is obtained. The step have been also actually carried out industrially.
By isomerizing the isomeric mixture of dichlorobenzenes, p-DCB, crude o-DCB, purified o-DCB and the like in the presence of a catalyst mainly composed of anhydrous aluminum chloride, an isomeric mixture of dichlorobenzenes rich in m-DCB (from 40 to 60% by weight of m-DCB, from 20 to 30% by weight of p-DCB and from 10 to 30% by weight of o-DCB) is obtained, and by distilling o-DCB off from the isomeric mixture, crude m-DCB consisting essentially of from 55 to 70% by weight of m-DCB, from 30 to 45% by weight of p-DCB and from 0 to 3% by weight of o-DCB is obtained (refer to G. A. Olah et al., J. Org. Chem., 27, 3449-3455 (1962) and U.S. Pat. No. 3,170,961).
The isomeric mixture of dichlorobenzenes according to the present invention is the above-mentioned crude o-DCB, purified o-DCB and crude m-DCB. According to the process of the present invention, purified o-DCB containing a smaller amount of p-DCB is obtained from the crude o-DCB or the purified o-DCB, and a highly pure m-DCB is obtained from the crude m-DCB.
Various processes have been proposed as the process for removing p-DCB from the crude m-DCB to obtain a highly pure m-DCB, however, every one of them uses a large amount of chemicals and separating agents, or consumes a large amount of energy and accordingly, the industrial value thereof is quite low. For instance, in the process by which the crystallization through deep cooling and the accurate distillation are used in combination (refer to DE-OS No. 2855940), crude m-DCB is cooled to a temperature of -30.degree. C. lower than the melting point of m-DCB (-24.degree. C.), thereby crystallizing the eutectic crystals of m-DCB and p-DCB, and thus removing p-DCB, then the still-remaining p-DCB is removed by the accurate distillation. However, since the difference of the boiling point of p-DCB (174.1.degree. C.) and that of m-DCB (173.0.degree. C.) is very small, a large amount of energy is necessary for carrying out the accurate distillation.
In the process of utilizing the property that only m-DCB is selectively sulfonated in the case of subjecting crude m-DCB to sulfonation by sulfuric acid (refer to Japanese patent publication No. 57-4614/1982), a large amount of sulfuric acid is used and a large amount of energy is consumed for hydrolysis of the sulfonated product and for regeneration of the spent sulfuric acid.
In the process of utilizing the property that only m-DCB is brominated in the case of subjecting crude m-DCB to bromination (refer to U.S. Pat. No. 3,170,961/1965), a mixture of p-DCB and m-DCB, the difference between the boiling point of (p-DCB) and that of m-DCB being small, is brominated to convert the mixture into the mixture of p-DCB (boiling point: 92.degree. C./58 mmHg) and 1-bromo-2,4-dichlorobenzene (boiling point: 141.degree.-142.degree. C./58 mmHg), and the thus treated mixture is distilled to remove p-DCB and 1-bromo-2,4-dichlorobenzene is reduced to m-DCB by hydrogen. However, the process is not economical because of the consumption of a large amount of expensive bromine.
In the extracting distillation process (refer to Japanese patent application Laying-Open (KOKAI) Nos. 50-19722/1975, 54-160322/1979, and 58-174336/1983, since an expensive extracting solvent such as hexamethylphophorotriamide, sulforane, alkylaniline, dimethylimidazolidinone, etc. is used, not only there are problems concerning the safety and the stability but also the separating performance is not sufficient and a large amount of energy is necessary.
In the process of removing p-DCB by adsorption on zeolite (refer to Japanese patent applications Laying-Open (KOKAI) No. 52-62229/1977, No. 58-131924/1983 and No. 58-150524/1983), the performance of zeolite in separating p-DCB from the mixture of p-DCB and m-DCB is not sufficient and a large amount of zeolite is necessary. In addition, a large amount of a solvent is necessary for desorbing the adsorbed material and a large amount of energy is consumed for recovering the solvent.
Although it has been known that (1) p-DCB and BCB, (2) p-DCB and DBB and (3) p-DCB and BCB plus DBB respectively form the eutectic crystals or the solid solution (refer to A. N. Cambell et al.; J. Am. Chem. Soc., 70, 553-561 (1948)), there have not been proposed to separate p-DCB from the isomeric mixture of dichlorobenzenes while utilizing the property of p-DCB in forming the eutectic crystals with BCB, etc.
As a result of the present inventors' studing on the solid-liquid equilibrium relationship of (1) p-DCB and BCB, (2) p-DCB and DBB and (3) p-DCB and BCB plus DBB in the solvent (m-DCB and o-DCB), it has been found that in the case of adding BCB, DBB or a mixture of BCB and DBB to the isomeric mixture of dichlorobenzenes and cooling the thus formed mixture, p-DCB is crystallized as the eutectic crystals with BCB, DBB or mixture of BCB and DBB, and the present inventors have attained the present invention based on the above-mentioned findings.